The present invention relates generally to bicycles, and more particularly to a rear suspension system which provides efficient energy transmission but still provides compliant suspension action when the bicycle is ridden over rough terrain.
Shock absorbing rear suspensions for bicycles are known. In general, however, these have not proven entirely satisfactory in practice.
In most rear suspension assemblies, the rear axle pivots about a single point when subjected to bump forces, as when traversing rough terrain. In these designs, the pedaling forces which are exerted by the rider tend to either compress or extend the spring/damper assembly of the rear suspension. In this respect, the spring/damper assembly of the rear suspension is affected by the pedal force and some of the rider""s energy is needlessly wasted.
This effect manifests itself by the common tendency of rear suspension systems to either lock up or xe2x80x9csquatxe2x80x9d when the rider pedals. Since most of these systems have a single lever arm which pivots about a single axis, the lock up or squat generally occurs as a result of chain tension acting on the single lever arm. If the single pivot line is above the chain line, the suspension will typically lock up and/or xe2x80x9cjackxe2x80x9d, thereby providing compliance only when the shock or bump force exceeds the chain tension. Conversely, if the single pivot point of the suspension system is below the chain line, the system will typically squat, since the chain tension is acting to compress the spring/damper assembly of the rear suspension system, similar to a shock or bump force.
The present invention has solved the problems cited above. Broadly, this is a bicycle comprising: a chain drive, in which the distance from the axis of a drive sprocket to the axis of a rear wheel hub is represented by a variable value CSL; and a compressible rear suspension having a linkage for moving the hub along a controlled wheel travel path as the suspension is compressed, the controlled wheel travel path having an arc radius which is greater towards a lower end of the path and smaller towards an upper end of the path.
The wheel travel path may comprise (a) a preferred pedaling position at a predetermined position Dp which is located along the rear travel path; (b) a lower curve segment below the position Dp in which there is an increasing rate of chainstay lengthening with increasing compression of the suspension system, such that the first derivative relationship       ⅆ          [      CSL      ]            ⅆ          (      D      )      
is a curve having a generally positive slope, so that the second derivative relationship             ⅆ      2        ⁢          [      CSL      ]                  (              ⅆ                  (          D          )                    )        2  
is generally positive; and (c) an upper curve segment above the position Dp in which there is a decreasing rate of chainstay lengthening with increasing compression of the suspension system, such that the first derivative relationship       ⅆ          [      CSL      ]            ⅆ          (      D      )      
is a curve having a generally negative slope, so that the second derivative relationship             ⅆ      2        ⁢          [      CSL      ]                  (              ⅆ                  (          D          )                    )        2  
is generally negative.
The linkage may comprise upper and lower link members which connect a rear frame section to a forward frame section. The link members are pivotally mounted to the frame sections, with the upper link member extending in a downward and forward direction when the suspension is in an uncompressed position, and the lower link member extending in a downward and rearward direction in this position. The link members are mounted so as to rotate in opposite directions as the suspension is compressed.
A shock absorber may be mounted between the lower link member and the forward frame section so as to be compressed with compression of the rear suspension. The lower end of the shock absorber may be mounted to a second arm of the lower link member.